If you get the black flickery screen when launching in Extended just power off and on the HMD. It occasionally happens to me since updating but it's a fast workaround. I've not seen any other problems.
Tried that yesterday was still getting double image. Saw where someone said on the Oculus forum that this load automatically makes your HMD the main monitor even though it does not say it did in display properties and that he played with that and got it working. I don't feel like playing around with it put it back to what works until?